1. Field of the Invention
The present invention relates to superhard metal-carbides, having a Moss hardness of from 9 to 10, made into solid (nonporous) microspheres with diameters of from 10 to 100 .mu.m. The microspheres are fabricated with the aid of high-temperature plasma-torch reactors from fully liquefied carbides of vanadium, niobium, tantalum or tungsten, fed into the torch in the form of a stream of discrete solid particles or mechanically preformed particle aggregates.
More specifically, the invention relates to the comminution of mineral and/or organic powders with the aid of ball-milling techniques employing the above metal-carbide microspheres as the grinding media. The comminuted particles, the coarsest of which are essentially 100%, by weight, finer than 0.9 .mu.m e.s.d. (equivalent spherical diameter) and the finest of which have diameters approaching or reaching 0.002 .mu.m (20 .ANG.), can be obtained in the form of narrow-particle-size-spread, or even nearly monodisperse populations.
2. Discussion of the Relevant Art
U.S. Pat. No. 3,909,241 to Cheney et al. discloses an improved process for a high-temperature plasma-torch agglomeration of finely divided metal powders into substantially spherical, dense particles to impart a free-flowing characteristic to the resultant agglomerate products.
Flame (plasma-torch) spraying techniques are widely used in the art to deposit on various industrial metal objects coatings formed from in-situ-fused metallic or ceramic powders. The function of these coatings is to increase the metal objects' wear and corrosion resistance, improve friction properties or reconstitute worn-out surface regions. The quality of the resultant coatings depends on how uniformly the metallic and ceramic powders are injected into the flame and how uniformly they become distributed within the latter before being deposited onto the target surface. To attain a high degree of uniformity, the fluidized powders must possess adequate free-flowing properties.
Since metallic and ceramic powders with particle diameters of less than 40 .mu.m perform rather unsatisfactorily in flame-spraying applications, it is preferred in the art to employ powders preagglomerated thermally into essentially spherical aggregates (microspheres) having diameters in excess of 40 .mu.m. It is important to point out, however, that plasma-torch agglomeration of metallic or ceramic powders into free-flowing microspheres, as practiced in the art, involves only a partial meltdown (fusion) of the particles fed into plasma-torch reactors. The fused portion of the thermally aggregated feed powders, which is confined essentially to the surface zone of the resultant microspheres, rarely exceeds 40% of the total powder mass. As a matter of fact, a complete plasma-torch meltdown of feed powders into 100%-solid, nonporous microspheres would make their deposition onto the target surfaces, as well as the formation of fused-on coatings, unduly cumbersome if not outright impractical. The reason for this is that fully melted-through, solid microspheres are much denser and require higher flame-spraying temperatures for remelting than the presently used partially melted-through, bulkier (porous) microspheres.